Science, Tech, Math › Science First Law of Thermodynamics Definition Chemistry Glossary Definition of First Law of Thermodynamics Share Flipboard Email Print Witthaya Prasongsin / Getty Images Science Chemistry Chemical Laws Basics Molecules Periodic Table Projects & Experiments Scientific Method Biochemistry Physical Chemistry Medical Chemistry Chemistry In Everyday Life Famous Chemists Activities for Kids Abbreviations & Acronyms Biology Physics Geology Astronomy Weather & Climate By Anne Marie Helmenstine, Ph.D. Chemistry Expert Ph.D., Biomedical Sciences, University of Tennessee at Knoxville B.A., Physics and Mathematics, Hastings College Dr. Helmenstine holds a Ph.D. in biomedical sciences and is a science writer, educator, and consultant. She has taught science courses at the high school, college, and graduate levels. our editorial process Facebook Facebook Twitter Twitter Anne Marie Helmenstine, Ph.D. Updated September 05, 2019 The first law of thermodynamics is the physical law which states that the total energy of a system and its surroundings remain constant. The law is also known as the law of conservation of energy, which states energy can transform from one form into another, but can neither be created nor destroyed within an isolated system. Perpetual motion machines of the first kind are impossible, according to the first law of thermodynamics. In other words, it is not possible to construct an engine that will cycle and produce work continuously from nothing. First Law of Thermodynamics Equation The equation for the first law can be confusing because there are two different sign conventions in use. In physics, particularly when discussing heat engines, the change in the energy of a system equals the heat flow in the system from the surroundings minus the work done by the system on the surroundings. The equation for the law may be written: ΔU = Q - W Here, ΔU is the change in the internal energy of a closed system, Q is the heat supplied to the system, and W is the amount of work done by the system on the surroundings. This version of the law follows the sign convention of Clausius. However, the IUPAC uses the sign convention proposed by Max Planck. Here, net energy transfer to a system is positive and net energy transfer from a system are negative. The equation then becomes: ΔU = Q + W Sources Adkins, C. J. (1983). Equilibrium Thermodynamics (3rd ed.). Cambridge University Press. ISBN 0-521-25445-0.Bailyn, M. (1994). A Survey of Thermodynamics. American Institute of Physics Press. New York. ISBN 0-88318-797-3.Denbigh, K. (1981). The Principles of Chemical Equilibrium With Applications in Chemistry and Chemical Engineering (4th ed.). Cambridge University Press. Cambridge UK. ISBN 0-521-23682-7.